The phenomenon of superconductivity was discovered by K. H. Onnes in 1911 when he demonstrated that the electrical resistance of mercury drops to zero when it is cooled to approximately 4 Kelvin (K). For many years the phenomenon of superconductivity remained a scientific curiosity, with few practical applications.
On March 2, 1987, M. K. Wu et al. published a paper entitled "Superconductivity at 93 K in a New Mixed-Phase Y-Ba-Cu-O Compound System at Ambient Pressure" in Physical Review Letters, Volume 58, Number 9. This announcement caused considerable excitement in the scientific and business communities because, for the first time, a superconductor material had been discovered which could use liquid nitrogen as a coolant. Since liquid nitrogen cooling systems are at least an order of magnitude less expensive than liquid helium cooling systems, many applications for superconductors suddenly became practical after Wu's discovery.
Standard processing of YBaCuO superconductor, as reported in the literature, results in a multi-crystalline solid comprising numerous, sintered-together superconducting grains. Measurements by many researchers in the field have indicated that the critical current (i.e. the maximum superconducting current of a material) that can be carried by this multi-crystalline structure is typically in the vicinity of 10.sup.3 amperes./cm.sup.2. This contrasts with a critical current of approximately 10.sup.5 amperes/cm.sup.2 in single crystal YBaCuO. Unfortunately, single crystals of this superconducting substance have, to date, only been made as thin-films or as crystallites having maximum dimensions of only a few millimeters.
Because the critical current in multi-crystalline YBaCuO is approximately two orders of magnitude less than the critical current in single crystal YBaCuO, it appears that the individual superconducting grains are insulated from each other. The insulation may result from inter-granular contamination, or may simply be empty space between the grains. In either event, the superconducting current must tunnel through the insulation between the superconducting grains, thus drastically reducing the critical current in multi-crystalline YBaCuO.